Such a timepiece is described for example in the U.S. Pat. No. 4,271,498. It comprises in this case an annular member of a piezoelectric material having one face fixed to the movement and having the other face fixed to the heel of the glass.
When an alternating electric signal is applied to the electrodes arranged on the piezoelectric annular member, the thickness of the latter varies causing displacements of the glass and the production of an acoustic wave in the air surrounding the timepiece. The electro-acoustic transducer formed by the movement of the timepiece, by this annular member and by this glass functions thus as a loud-speaker.
When on the contrary an acoustic wave puts the glass under an alternating pressure, the displacements of this glass cause variations of the thickness of the piezoelectric annular member which provoke the appearance of an electric signal between the above-mentioned electrodes. In this case, the electro-acoustic transducer functions as a microphone.
In such a transducer, the amplitude of the displacements of the glass is clearly always identical to the variations of the thickness of the piezo-electric annular member.
As a result, when the transducer functions as a loudspeaker, the amplitude of the displacement of the glass is weak, so that the sound produced by these displacements will also be weak.
In a similar manner, when the transducer functions as a microphone, the amplitude of the electric signal produced in response to the displacements of the glass will also be weak.